This study will explore the local organization of monoaminergic dendrite bundles. The morphological characteristics, relationships, and transport capabilities of specific neuronal elements and supporting tissues within dendrite bundles in the medullary raphe nuclei and the locus coeruleus of the developing and adult rabbit brain will be examined. The specific elements of the dendrite bundles to be explored are monoamine dendrites, non-monoamine dendrites, shafts from fourth ventricular tanycytes, and capillaries which directly abut neuronal perikaryal and dendritic membranes. Three major studies will be undertaken: (1) Morphological characteristics, subcellular specializations, and relationships of the dendrites, tanycyte shafts, and capillaries in the dendrite bundles will be explored with a Golgi-Cox impregnation technique, a cresyl violet stain, and fluorescence histochemistry at the light microscopic level, and with routine electron microscopic embedment and the en bloc phosphotungstic acid method at the electron microscopic level; (2) Localization and identification of monoamines in these bundles will be achieved by fluorescence microscopy and X-ray analytical electron microscopy; (3) Uptake and transport capabilities of fourth ventricular tanycytes sending shafts into the monoaminergic dendrite bundles will be determined by the horseradish peroxidase method using the tetramethylbenzidine reaction and by light microscopic autoradiography of radiolabeled dopamine, 5-hydroxytryptophan, and thyrotropin releasing factor (TRF). These studies will establish the morphological substrate for local interactions within monoaminergic dendrite bundles and for possible delivery of CSF-borne and blood-borne substances to monoaminergic dendrite bundles. This morphological substrate must be examined and characterized before further physiological studies of these bundles can be undertaken. Local dendritic influences and fluid-borne influences could determine the local excitability of the serotonergic and noradrenergic systems of the C.N.S., widely implicated in sensory, motor, visceral, and cognitive functions and disorders.